Conventionally, in order to purify exhaust gas from an internal combustion engine, an emission purifying catalyst unit (three-way catalyst unit) is disposed in an exhaust passage, and feedback control is performed on the basis of an air-fuel ratio detected by means of an air-fuel ratio sensor provided in the exhaust passage in such a manner that the air-fuel ratio of an air-fuel mixture supplied to an engine attains a stoichiometric air-fuel ratio (theoretical value). Thus, nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (HC) are reduced concurrently. Accurate performance of the above-mentioned feedback control is effective for improving the degree of purification of exhaust gas from an internal combustion engine.
Moreover, in an effort to further improve the exhaust purifying performance of an emission purifying catalyst unit, studies have been conducted on a method for effectively utilizing an oxygen storing function (action) of the emission purifying catalyst unit. In this method, when a to-be-removed substance contained in exhaust gas is oxidized or reduced, oxygen stored in the emission purifying catalyst unit or the oxygen storing function of the emission purifying catalyst unit is utilized.
A control apparatus disclosed in Japanese Patent Application Laid-Open (kokai) No. H5-195842 is an apparatus designed so as to utilize such an oxygen storing function. Specifically, the apparatus is designed so as to estimate the amount of oxygen stored in the entirety of an emission purifying catalyst unit (oxygen storage amount, actual charge level of oxygen) on the basis of a product of an intake-air flow rate, an oxygen content of air, and a deviation from the lambda value 1 (stoichiometric air-fuel ratio), and to control the air-fuel ratio in such a manner that the oxygen storage amount attains a fixed target value.
However, the above-described control is adapted to maintain the oxygen storage amount at a level in the vicinity of the target value, and thus, has a problem of being unable to perform air-fuel ratio control for reducing emission of one or more specific components of exhaust gas to a desired level or less, or air-fuel ratio control for maintaining, to a level near the desired level, the oxygen storage amount stored in an upstream portion of a catalyst unit, rather than the oxygen storage amount stored in the entire catalyst unit.
In view of the foregoing, the present inventor has developed a method for estimating the emission of a specific component of exhaust gas (or a representative value representing a state of the exhaust gas) that flows out of an emission purifying catalyst unit (and that is purified by the emission purifying catalyst unit); and has found that the exhaust purifying performance of the emission purifying catalyst unit can be improved through performance of air-fuel ratio control in such a manner that the estimation value attains a target state.